Abstract

Threshold-elevation (TE-) versus-mask-spatial-frequency (SF) curves and TE-versus-mask-contrast curves, produced by the oblique-masking technique, were reported for uncrossed stimuli (color-test-on-color-mask and luminance-test-on-luminance-mask) [Invest. Ophthalmol. Visual Sci. Suppl. 34, 751 (1993) and Vision. Res. 23, 873 (1983)]. The technique minimizes the artifacts that are due to spatial phase effects, spatial beats, spatial probability summation, and local cues. My goal was to measure these curves for crossed stimuli (color-test-on-luminance-mask and luminance-test-on-color-mask) by this oblique-masking technique and to compare the curves with those reported in previous studies. For this purpose threshold contrasts were measured by a yes–no procedure with randomized double staircases. Test targets were vertical spatially localized (D6) patterns, and masks were oblique sinusoidal patterns; both the test and the mask were presented simultaneously, for 2 s (Gaussian window), on a color monitor interfaced with an ATVista system and a Powell achromatizing lens. The test SF’s were 0.125, 0.5, 2, 4, and 8 cycles per degree (cpd); mask SF’s were 0.031–16 cpd; and mask contrasts were 6.25%–50%. Furthermore, the Red–Green channel was defined by the minimum flicker and the hue cancellation techniques. Results show mostly masking effect (TE>1) at contrasts above threshold; sometimes, separability (TE=1) and above-threshold facilitation (TE<1) effects were also observed, depending on the test SF, the mask SF, the mask contrast, and the subject. In general, the magnitudes of TE’s are smaller and the TE-versus-mask-SF curves are slightly narrower for the oblique-cross-masking conditions than those for the respective oblique uncross masking. In addition, the TE-versus-mask-contrast curves for the crossed conditions are mostly shallower than those for the respective uncrossed conditions. Furthermore, mostly the color–luminance asymmetry (color masks luminance more than luminance masks color) is found, in mild form, for SF’s⩾0.5cpd. For the lower SF of 0.125 cpd, there is either a lack of asymmetry or a very mild asymmetry of the opposite kind (luminance masks color slightly more than color masks luminance) seems to prevail. In general, the oblique-masking data shows mild asymmetry and reduced facilitation; both are consistent with reduced local cues, similar to those shown by randomized phase data, thus making the data suitable for SF analysis; moreover, at high contrast, the masking data are consistent with those reported in previous studies.

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